1. Field of the Invention
This invention relates to the making of polypropylene ether glycol compositions, and in particular, to the making of such compositions having a relatively high molecular weight, such as approximately 3,500 to about 100,000.
2. Description of the Prior Art
It is known that in the making of polymers containing chains of oxypropylene units by base-catalyzed oxypropylation, there is a limit to the average molecular weight which can be achieved. When the chain of oxypropylene units becomes longer than about 30 or more units, there is an increasing tendency for a "transfer" reaction to occur in place of the desired chain-growing or "propagation" reaction. The "transfer" reaction creates an allyl-alcohol entity which is relatively more reactive than the polymer, and it, or oxypropylated polymers derived from it, compete with increasing success for the available unreacted propylene oxide. See, for example, "Polyethers" by C. C. Price, Accounts of Chemical Research, Vol. 7, p. 294 and 295, (1974). The art has lacked a good way of making oxypropylated polyether compositions of advanced molecular weight.
It can be taken as known that the heating of esters of benzenesulfonic acid in the presence of excess alcohol yields ethers (Witmore, Organic Chemistry, 2nd edition, D. Van Nostrand Company, Inc., New York, 1951, p. 628). Moreover, the benzenesulfonic acid diester of ethylene glycol is known from Foldi, Berichte, 60, 660 (1927), but the concept of using such compound or another difunctional arylsulfonic acid ester as a coupling agent to overcome the molecular-weight limitation in the making of polymers containing chains of oxypropylene units is believed to be novel.
For some purposes, it is particularly desirable to obtain a composition which is substantially 100% polyether polyol; unlike materials containing ester linkages, such materials are not subject to being degraded by hydrolysis when they are present in a strongly acidic or strongly basic aqueous medium. For such uses, it is not possible to obtain a still-higher molecular weight by the use of a difunctional carboxylic acid as a coupling agent, since such an acid will ordinarily yield ester linkages.